The invention relates to an X-ray examination apparatus for forming X-ray images of an object, which apparatus includes
an X-ray source for generating an X-ray beam, PA1 an X-ray filter which is provided with filter elements which are arranged to contain an adjustable quantity of X-ray absorbing liquid in order to adjust an intensity profile on an object, and with a supply duct for connecting the filter elements to a reservoir for the X-ray absorbing liquid, PA1 an X-ray detector for receiving a part of the X-ray beam, having traversed the object, in order to detect an X-ray image.
The invention also relates to an X-ray filter for use in an X-ray examination apparatus of this kind.
An X-ray examination apparatus of the kind set forth is known from international patent application WO 96/13040. The X-ray filter in the known X-ray examination apparatus is used to limit the dynamic range of an X-ray image of an object which is formed on the X-ray detector, for example a human or animal body to be examined. The filter elements of the X-ray filter are constructed as capillary tubes, one end of which communicates with the X-ray absorbing liquid present in the reservoir. The X-ray absorbing liquid contains, for example aqueous solutions of salts of, for example lead, cesium or tungsten. The quantity of X-ray absorbing liquid can be adjusted by way of electrowetting. To this end, the tubes are provided with an electrical conductor which serves as an electrode. Furthermore, an electrically insulating coating layer is provided on the electrode. In the context of the present application the term "electrowetting" is to be understood to mean an adjustable adhesion of the X-ray absorbing liquid to the electrically insulating coating layer, which adhesion is dependent inter alia on the value of an electric voltage applied across the electrically conductive layer and the X-ray absorbing liquid. As a result, the filling of each of the capillary tubes can be adjusted by variation of the electric voltage value so that an X-ray absorption profile of the X-ray filter is adjusted within a short period of time, for example 0.4 seconds.
In order to form an image of the desired organs in the object, the X-ray detector is arranged opposite the X-ray source on a first axis with a part of the object to be imaged, and the X-ray filter is situated on this first axis between the X-ray source and the object, an entrance face of the X-ray filter then being oriented transversely of the first axis.
The arrangement is functional if the first axis is directed vertically during operation. It is a drawback of the known X-ray examination apparatus that, when the first axis is directed horizontally, the adjustment of the quantity of X-ray absorbing liquid in the capillary tubes is susceptible to an uneven hydrostatic pressure distribution in the supply duct.